The present invention relates generally to power supplies utilizing transformer isolation and requiring very fast isolated voltage turn-off, such as may be useful in electron tube applications, such as x-ray tubes or electron beam guns, for example.
As an exemplary application, gridding circuits for controlling x-ray tubes reduce radiation exposure associated with decaying x-ray tube current. For fast x-ray beam turn-on and turn-off, the gridding voltage must be applied and removed rapidly. In typical gridding circuit arrangements, the grid voltage turn-off (i.e., x-ray beam turn-on) is accomplished by discharging an output capacitor through a bleeder resistor. Due to the time needed to discharge the output capacitor, as well as a practical limitation on the power loss allowable in the bleeder resistor, the output capacitance value is limited. As the output capacitance is limited, a high-gain path is formed between the output capacitance and the parasitic capacitance of the grid with respect to ground for the ac ripple from the high voltage generator. The modulation caused by the ac ripple across the grid and cathode during x-ray exposure tends to shorten filament life, and hence tube life, and also tends to degrade image quality.
Low output impedance gridding circuits are available but are limited in dynamics, e.g., by the time required to reset or discharge the parasitic impedances introduced by the isolation transformer magnetizing inductance and winding capacitance. Other low impedance gridding circuit arrangements disadvantageously require that the power control and gating arrangement be supplied from the filament drive, typically resulting in reliability problems.
Accordingly, it is desirable to provide a simple gridding circuit with a low output impedance which does not restrict grid dynamics, e.g., in terms of time and speed. It is furthermore desirable to provide a gridding circuit which overcomes reliability issues associated with existing gridding circuit arrangements.